The present invention relates to an apparatus for checking atmospheric pollution, which is more particularly applicable to atmospheres polluted by alpha particle emitting aerosols.
The invention is more particularly applicable to the case where the aerosols contain plutonium, uranium, curium or other elements, whereof the energies of the alpha particles are generally below 6 MeV. When it is wished to detect such aerosols, the decay products of radon isotopes contained in the atmosphere and which are themselves alpha particle emitters, are highly prejudicial to the measurements, particularly radium A (Po-218) and thorium C (Bi-212), which have an emission energy of 6 MeV, radium C' (Po-214) having an emission energy of 7.7 MeV and thorium C' (Po-212) having an emission energy of 8.8 MeV.
The conventional method used for detecting these aerosols consists of sucking air through a filter, which stops the aerosol. This filter is placed in front of a spectrometric detector associated with amplitude selectors making it possible to discriminate the emissions due to the radon from those due to the aerosols in question. The often circular detector is positioned parallel to the filter and facing the same, on the side where the aerosols are trapped and at a distance of a few millimeters. This space is necessary to permit the passage of the gaseous stream carrying the aerosols. The energy of the alpha particles emerging from the filter towards the detector is partly absorbed by the air. Certain particles follow a path which is perpendicular to the filter and detector, whilst others follow a more or less sloping path. Thus, for a given emission energy, the alpha particles are detected with an energy which becomes smaller the longer the path which they have followed in the air, i.e. the more it is inclined with respect to the direction perpendicular to the filter and the detector. This energy loss leads to a superimposing of the "degraded" energies of the alpha particles of the decay products of the radon isotopes and the lower energies of the alpha particles of the isotopes which it is wished to detect.
FIG. 1 is a graph giving the number N of alpha particles detected as a function of the emission energy E. This graph shows two peaks, one of which corresponds to the alpha particles of the radium C', whilst the other corresponds to the alpha particles of the radium A and thorium C. Those particles have undergone an energy loss by interaction with air. Their remaining degraded energy is of the same order of magnitude as that of the alpha particles of e.g. plutonium 238 and is superimposed on the energy peak of the latter. The hatched area A to the left of the peak corresponding to the radium A and thorium C corresponds to spurious pulses due to the radon in the window useful for detecting plutonium 238.
Two solutions have been provided for obviating this disadvantage. The first consists of arranging a grid with closed cells between the filter and the detector for stopping the alpha particles which do not follow a direction quasi-perpendicular to the latter. Thus, such an apparatus is illustrated in FIG. 2, where it can be seen that it comprises a filter 2 shaped like a flat disk and a detector 4 having a circular shape and having a planar detection face 6 facing filter 2. Between filter 2 and face 6 of detector 4 is arranged a grid 8' having a certain number of cells 10. The grid whose periphery is closed by plates perpendicular to the filter and detector, each cell being open, on the one hand, on the side of the filter and, on the other, on the side of the detector. Generally, the cells have a square cross-section, the side length being approximately 1 mm, whilst the total grid thickness is generally a few millimeters. Although this apparatus stops the alpha particles following an inclined path with respect to the direction perpendicular to the filter and detector, it suffers from the disadvantage that in order to permit the passage of the gaseous stream carrying the alpha particles, it is necessary to provide a space between the filter and the grid. This leads to a by no means negligible spacing between the filter and detector, which causes a supplementary energy loss of alpha particles of aerosols trapped on the filter and which it is wished to detect.
Another method consists of using algorithms for evaluating the number of spurious alpha particles and they are eliminated by removal from the recorded spectrum. However, this method leads to imprecise results.